Abstract

A cache-based computer system employs a fast, small memory -the " cache" - interposed between the usual processor and main memory. At any given time the cache contains as much as possible the instructions and data the processor needs; as new information is needed it is brought from main memory to cache, displacing old information. The processor tends to operate with a memory of cache speed but with main memory cost-per-bit. This configuration has analogies with other systems employing memory hierarchies, such as "paging" or "virtual memory" systems. In contrast with these latter, a cache is managed by hardware algorithms, deals with smaller blocks of data (32 bytes, for example, rather than 4096), provides a smaller ratio of memory access times (5:1 rather than 1000: 1), and, because of the last factor, holds the processor idle while blocks of data are being transferred from main memory to cache rather than switching to another task. These are important differences, and may suffice to make the cache-based system cost effective in many situations where paging is not.